Stents are generally cylindrically shaped devices that are radially expandable to hold open a segment of a blood vessel or other anatomical lumen after implantation into the body lumen. Stents have been developed with coatings to deliver drugs or other therapeutic agents.
Various types of stents are in use, including balloon expandable and self-expanding stents. Balloon expandable stents generally are conveyed to the area to be treated on balloon catheters or other expandable devices. For insertion, the stent is positioned in a compressed configuration along the delivery device, for example crimped onto a balloon that is folded or otherwise wrapped about a guide catheter that is part of the delivery device. After the stent is positioned across the lesion, it is expanded by the delivery device, causing the stent diameter to expand. For a self-expanding stent, commonly a sheath is retracted, allowing expansion of the stent.
Stents are used in conjunction with balloon catheters in a variety of medical therapeutic applications including intravascular angioplasty. For example, a balloon catheter device is inflated during PTCA (percutaneous transluminal coronary angioplasty) to dilate a stenotic blood vessel. The stenosis may be the result of a lesion such as a plaque or thrombus. After inflation, the pressurized balloon exerts a compressive force on the lesion thereby increasing the inner diameter of the affected vessel. The increased interior vessel diameter facilitates improved blood flow. Soon after the procedure, however, a significant proportion of treated vessels re-narrow or collapse.
To prevent acute vessel narrowing or collapse, short flexible cylinders, or stents, constructed of metal or various polymers are implanted within the vessel to maintain lumen size. The stent acts as a scaffold to support the lumen in an open position. Various configurations of stents include a cylindrical tube defined by a mesh, interconnected stents or like segments. Some exemplary stents are disclosed in U.S. Pat. No. 5,292,331 to Boneau, U.S. Pat. No. 6,090,127 to Globerman, U.S. Pat. No. 5,133,732 to Wiktor, U.S. Pat. No. 4,739,762 to Palmaz and U.S. Pat. No. 5,421,955 to Lau. Balloon-expandable stents are mounted on a collapsed balloon at a diameter smaller than when the stents are deployed. Stents can also be self-expanding, growing to a final diameter when deployed without mechanical assistance from a balloon or like device.
Stents have been made of various materials, including various metals and polymers. Various metals and alloys, such as stainless steel and MP35N have been successfully used as stent materials. However, no material is a perfect stent material. Each has its particular advantages and disadvantages. Therefore, some recent stents have attempted to combine the advantageous properties of different materials by laminating layers of different material to form the struts of a stent. Such stents are known as “laminated stents”. For example, Abbot Laboratories is marketing a product known as the TriMaxx® stent that is a laminated metal stent. In particular, the TriMaxx® stent includes a layer of tantalum sandwiched between two layers of stainless steel.
The circumferential surface of a strut of a laminated metal stent design includes different materials. These different materials can pose difficulties in coating such a laminated stent with, for example, a drug eluting polymer. Further, these different materials may provide a driving force for galvanic corrosion of the stent and present non-homogeneous mechanical properties, such as flexibility characteristics. Still further, the different layers of a laminated stent may come apart or de-laminate over time on some portions of the stent.